3rd International EuroConference on High-Performance Marine Vehicles HIPER'02, Bergen, 14-17 September 2002, pp 392 - 406

Manoeuvring Aspects of Fast Ships with Pods

Serge Toxopeus, Maritime Research Institute Netherlands, S.L.Toxopeus@Marin.nlGiedo Loeff, Maritime Research Institute Netherlands, G.Loeff@Marin.nl

Figure 1. State of the art cruise liner with podded propulsion

Abstract

Currently an increasing number of modern ships is equipped with podded propulsors. Advantages arethe possibility to use a diesel-electric propulsion plant and to increase the propulsive efficiency andthe manoeuvrability of the ship. Besides application to cruise ships the pod concept is nowadays alsoapplied to other ship types and to vessels of a higher speed range. Major benefits are normallyobtained, but still areas of special attention exist. Due to the possibly large steering forces, largesteering-induced heeling angles can occur. Additionally, the directional stability of ships with podstends to be less than comparable ships with conventional propulsion. This paper describes the aspectsof application of pods from a manoeuvring viewpoint, compares the manoeuvrability between shipdesigns with conventional propulsion and pod propulsion and highlights the benefits and points ofattention. Design guidelines to improve the manoeuvring performance are given and operationalissues are discussed.

1 Introduction

For the past ten years, an increase of the use of pod propulsion is discerned. The reason for this isclaimed to be the increase in propulsive efficiency, comfort and increased manoeuvrability. Especiallyships with diesel-electric propulsion appear to gain from this concept. Other advantages are theincreased flexibility in engine room layout and the location of the engine room along the ship.

Currently, investigations on the design of pods are made from both a structural point of view as wellas a hydrodynamics point of view. In the market several designs are spotted varying in number ofpropellers and general outline. Main suppliers are ABB Azipod, Rolls-Royce, Siemens-Schottel and

Wärtsilä Propulsion. From a structural point of view the accommodation of the electric motor, themounting of the pod and the overall strength of the pod body are decisive for the design.Information gathered from the Co-operative Research Ships (NSMB-CRS) studies and in-housemeasurements conducted at MARIN, points out that significant improvements are still to be made inthe hydrodynamic design regarding the propulsion and steering efficiency of pods. General designaspects that are under investigation are:� Torpedo and strut design� Design and placement of fins attached to the pod housing� Steerable flaps for course keeping� Optimisation of orientation of the pod unit in all directions (lateral - transverse -horizontal plane)

Classically, the first cruise ships equipped with pods had two units installed. The largest shipsdelivered currently however have one fixed unit and two azimuthing units due to the current limitationof the power of the pods. The largest ship on order will be equipped with two fixed pods and twoazimuthing pods. Using fixed pods poses a special challenge from a manoeuvring viewpoint: it meansthat only part of the total installed power will be available for steering and therefore a relativelyinsufficient steering ability might exist, in contrast to the ships with all units movable.

Several publications regarding the general optimisation of ships equipped with pods have beenreleased, by for example Kurimo (1998), Lepeix (2001) and Hämäläinen (2001). However, thesepublications deal mostly with general issues or concentrate on the powering optimisation of the ship.Only marginal information regarding the manoeuvring specifics is found in public literature.

Obviously, considerable advantages exist from a manoeuvring viewpoint when pods are implementedin the design of the ship. However, without taking the appropriate measures, the directional stabilityof the ship or even the safety of the ship might be compromised due to the hull form design or thelarge steering forces of the pods. These measures, which are most of the time easily implemented,must be taken in the earliest stages of the design.

When judging the manoeuvring characteristics of ships, not only the basics as proposed by regulatoryinstitutions should be verified, but also characteristics associated with the mission of the ship. Forexample, passenger ships should not obtain large heeling angles during manoeuvres and they shouldbe able to manoeuvre without assistance in harbours. Therefore, these aspects should be studiedduring the design of the ship in order to guarantee the success of the ship.

This paper presents the characteristic features regarding the manoeuvrability of ships with pods andprovides guidelines for the designer in order to avoid the disadvantages associated with pods.

2 Differences between conventional ships and pod ships

The hull lines of ships with pods are slightly different from comparable ships with conventionalpropulsion. To allow the pods to rotate 360°, the pods must be mounted at a flat surface of the hull.Additionally, because of strength considerations, the pods are presently only positioned with the strutvertically down when looking from behind. This means that the hull lines are very flat in the aft shipand that not much lateral area exists in the aft ship, compared to conventional ships. For sufficientdirectional stability and for docking a suitable centre line skeg is required. In Figure 2 and Figure 3,typical aft ship designs of respectively a conventional ship and a ship with pods are given. In thesephotographs, the difference in the aft ship hull lines is clearly seen: the V-shaped lines for theconventional ship against the pram shaped lines for the ship with pods. The necessity of pram shapedlines for vessels equipped with podded propulsion is questionable. If desired, more V-shaped sectionscould also be implemented in the ship design if compromises are made to the design of the pod.Currently new pod designs are entering the market which not necessarily require pram shaped lines.

It is however expected that the most optimum hull form to accommodate a podded propulsionarrangement has not been found yet. When the horse-drawn cart was replaced by the self-propelledvehicle, the first design still resembled a cart. The evolution towards the modern day vehicles endurednumerous design changes, but it took a long time before all opportunities were explored. This analogyrepresents a typical trajectory of a technical evolution. The design of pods and the way ofimplementation in the ship-design are still in the course of evolution.

Figure 2. Typical optimised aft ship design of a conventional cruise vessel with twin propeller/twinrudder arrangement

Furthermore, the use of pods in the aft ship cancels the need for stern thrusters, but still increases theforces that can be generated while manoeuvring at low speeds. This is a major advantage of theapplication of pods. Additionally, because of the relatively large installed power of the pods comparedto stern thrusters, it means that in almost all cases sufficiently large forces can be generated in the aftship during crabbing operations. This leaves only the bow thrusters as the limiting factors in thecrabbing ability of the ship.

When course keeping during transit, application of steering angles is required to keep the ship on therequired course. For a ship with pods, this is done by rotating one or all of the pods. Because of theweight of the complete pod unit, this is considered to be more strainful for the equipment than whensteering with rudders. Therefore pod manufacturers are currently contemplating using steerable flapsconnected to the pod which may be used for course keeping. Alternatively, additional rudders forcourse keeping purposes have been studied in the past. However, the success of these additionalrudders were limited.

3 Manoeuvring at speed

Research conducted on vessels outfitted with podded propulsion has gained valuable informationabout the merits and drawbacks related to the manoeuvring characteristics at speed. From free sailingexperiments the general behaviour has been extensively evaluated. Several systematic researchprojects have been conducted on vessels equipped with either a conventional propeller-rudderpropulsion arrangement or a podded propulsion arrangement in order to identify the best conceptsuitable to the vessel under consideration. For example, models used during such a study are

presented in Figure 2 and Figure 3. Some experiments consisted of captive measurements monitoringthe forces and moments in all 6 degrees of freedom acting on the pod unit and on the hull. Insight is obtained by conducting these experiments regarding the general behaviour of a vesselpropelled by pods and detailed information of the forces working on the pods. Based on thisknowledge an assessment of the manoeuvring characteristics at speed for high-speed vessels equippedwith podded propulsion can be made. Other possible fruitful evolutions, such as hybrid propulsionarrangements and unconventional hull forms, will be interesting when disputing the application ofpodded propulsion in the design of high performance vessels.

Figure 3. Typical optimised aft ship design of a cruise vessel with podded propulsion and steeringarrangement

3.1 Areas of interest

In order to identify the merits and drawbacks of the manoeuvring characteristics related to theimplementation of podded propulsion in the design of high performance vessels, a few areas ofinterest will be discussed:� Course keeping� Turning and turn initiated roll� Course keeping in waves

3.2 Course keeping

In order to sail in a safe and efficient way a vessel needs to have good course keeping capabilities. Again in propulsion efficiency when applying podded propulsion could be of less importance when thecourse keeping behaviour reduces significantly. More effort (fuel) could be needed to sail a certaintrajectory. Therefore it is very important to look at the course keeping capabilities of ships equippedwith pods and especially in relation to conventional propeller-rudder configurations.

A vessel's course keeping ability in calm water is commonly benchmarked using standardmanoeuvres, see for example reference IMO (1994). These standard manoeuvres are regularlyconducted on model scale and full scale and can therefore be used to correlate a vessel with similar orother types of vessels. The International Maritime Organisation (IMO) proposed criteria for

parameters derived from the standard manoeuvres. These criteria, described in IMO ResolutionA.751(18) (1993), are commonly used to judge the manoeuvring characteristics of a vessel. Althoughcomparing the manoeuvring performance with that of other podded vessels might be more sensible.

The parameters derived from standard zigzag manoeuvres identify the course changing and coursechecking ability of a vessel. Statistical data have been derived from a selection of ships of which datais available at MARIN and is presented in Figure 4. It should be remarked that the presented vesselsare optimised concepts through extensive research.

1st overshoot 10°/10°

0

10

20

30

0 10 20 30 40L/V [s]

[deg

]

IMO

1st overshoot 20°/20°

0

10

20

30

0 10 20 30 40L/V [s]

[deg

]

Figure 4. Overshoot angle statistics of ships with podded propulsion

The overshoot angles presented in Figure 4 are well within the criteria proposed by the IMO and arecomparable to the average overshoot angles for all types of vessel in the MARIN database. From thisit can be concluded that the tested vessels with podded propulsion perform well regarding the yawchecking and course keeping ability. Data derived from systematic research on vessels with either conventional or podded propulsionenable a qualitative as well as quantitative comparison between both propulsion configurations.Figure 5 presents the overshoot angles for several comparable vessels, different approach speeds andsteering angle / yaw check angle combinations. The meanline represents the situation where theovershoot angles for both steering units are equal.

Overshoot angles

0

8

16

24

0 8 16 24� [deg], conventional units

� [d

eg],

pod

units

Meanline

Figure 5. Overshoot angles of ships equipped with podded or conventional propulsion units

The vessels with the pod units tend to have slightly larger overshoot angles than those with theconventional propeller-rudder units. This tendency cannot be ascribed to a difference in propulsionunits alone, other possible causes have to be questioned as well:

� Difference in rate of application of rudders or pods� Difference in GM value� Difference in the aft hull shape

Without questioning these aspects and judging their influence, no direct comparison between thesteering units can be made.

According to the classification society and SOLAS requirements, the rate of application of pods has tofulfill the requirements for azimuthing thrusters to be at least 9 degrees per second whereas the rate ofapplication of a rudder must be at least 2.32 degrees per second. In principle this requirement can beseen as a significant difference between both propulsion units. Increasing the rate of application of therudders or pods influences the steering behaviour by speeding up the course changing and coursechecking ability. A kind of 'equilibrium' of both phenomena will result in different overshoot angles.A difference in rate of application of the rudders or pods can also influence the roll behaviour of thevessel, relating to the natural period of roll. This steering rate will be further discussed in section 3.3,as well as the influence of the GM value.

The influence of the afthull shape on the course keeping ability can be best discussed based on themeasurement conducted with the pair of comparable vessels, shown in Figure 2 and Figure 3. Thefigures show a typical V-shaped aft ship for the conventional propeller rudder configuration andtypical pram shaped aft ship for the podded propulsion configuration. From research it is known thatthese typical afthull shapes differ in their dynamical coursestable behaviour. It was derived frommeasurements that a V-shaped aft ship tends to have a better dynamical course stability than theextreme pram shaped aft ship. Knowing this the application of pram shaped lines in the design of avessel with podded propulsion should be disputed. The compromise between optimum resistance,manoeuvring and seakeeping qualities will be dependent on the type, mission and application ofvessel.

Another critical aspect of the application of pods is the supposed cavitation behaviour of a pod andpropeller in an oblique flow. This cavitation issue will be very much of interest when evaluating theapplicability of pods in the design of high performance craft. Pustoshny and Karprantsev (2001)presented and commented on cavitation observations on the Elation passenger cruiser. Their findingsrelevant to the manoeuvring characteristics of high performance craft can be best summarised in thefollowing statements:

1. Pulling propellers on pods are normally exposed to an uniform wakefield, favouring goodcavitation characteristics and reduce propeller-induced pressure fluctuations and vibrations whilethe vessel is sailing in a straight line without helm.

2. Angles of incidence larger than 5°-7° while course keeping become critical concerning cavitation.3. The cavitation risk in a constant turn, for example a turning circle, is extensive. The speed in a

constant turn will drop significantly due to large drift angles and yaw rate which are excited bylarge steering forces. Speed reduction yields an overloaded propulsion condition contributing tothe risk of cavitation. The influence of the overloaded condition tends to be more significant thanthe influence of moderate inflow angles on the cavitation characteristics.

The application of pods in the design of vessels sailing very fast, say over 30 knots, is virtuallyunexplored. Cavitation on the propellers and on the pod houses due to strong propeller inducedtangential and radial velocities is supposed to be critical and requires thorough research anddevelopment. Since cavitation inception speed is of utmost importance for high performance ships,application of pods must be studied thoroughly because of the more extreme working regimes towhich the pods will be exposed and the subsequent larger reduction of cavitation inception speedduring manoeuvres compared to conventional propelled high performance ships.Alternatively the application of other control surfaces or units should be further explored. Steerableflaps at the trailing edge on the strut and additional rudders have been investigated concerning theirapplicability. The purpose of these additional control surfaces during course keeping at high speeds ispreventing the oblique inflow angles on the pods and propellers and excessive use of the pod'ssteering gear. Experiments and calculations have been conducted with these alternative steeringmethods. However, both concepts, the steerable flap and also the additional rudders, were up to nownot very fruitful.

Based on the discussion of the cavitation issue it is judged that this topic should be evaluated withabsolute care. Further research and development will be required to address all observations andsolutions should be studied further.

3.3 Turning and turn initiated roll

The standard IMO turning circle manoeuvre identifies the turning ability of a vessel. Statistical datahave been derived from a selection of ships with podded propulsion of which information is availableat MARIN and is presented in Figure 6. It should be remarked that the presented vessels are optimisedconcepts through extensive research.

Advance

0

2

4

6

0 10 20 30 40Steering angle [deg]

AD

/LPP

[-]

IMO

Tactical diameter

0

2

4

6

0 10 20 30 40Steering angle [deg]

TD/L

PP [-

]

Figure 6. Turning circle statistics of ships with podded propulsion

The turning circle data presented in Figure 6 shows that the advance and tactical diameter are wellwithin the criteria proposed by the IMO. Steering angles were limited to 35 degrees during thepresented tests, however the criteria as proposed by the IMO apply to the largest possible steeringangles. The turning ability of a podded propelled vessel will therefore be even better than shown inFigure 6.Based on the experiments conducted with models equipped with either a conventional propulsion unitor a podded propulsion unit, a comparison can be made concerning their inherent turning ability.Figure 7 presents a comparison between the turning circle data of the two propulsion configurations.The meanline represents the situation when the values for both steering units are equal.

Turning circle diameter

0

1

2

3

4

0 1 2 3 4Dstc/LPP, conventional units

Dst

c/LPP

, pod

uni

ts

Meanline

Tactical diameter

0

1

2

3

4

0 1 2 3 4TD/LPP, conventional units

TD/L

PP, p

od u

nits

Meanline

Figure 7. Turning circle data of ships equipped with podded or conventional propulsion units

It clearly shows that the turning ability of the vessels with the podded propulsion is better than thevessels with the conventional propeller-rudder arrangement. Clarifications for this superior turningability can be ascribed to larger steering forces generated by the pod units, see Figure 11, and thelarger speed loss. As a result of these large steering forces, larger drift angles and high speed losswere measured in the steady turning circle.

Turning ability itself is clearly not a problem when judging the applicability of podded propulsion. Incurrent research, the roll behaviour while manoeuvring is the centre of attention. Especially for highspeed vessels and vessels with a low GM value, heel is of importance. The effect works in two ways:� High turning rate can cause large gyration forces and thus large roll motions� Heel angles effect the turning rate and the course stability

The effect of roll motions on the manoeuvrability of a vessel has been studied and presented by forexample Son and Nomoto (1981), Oltmann (1993) and Kijima and Furukawa (1998).

The importance of this issue related to the application of podded propulsion can be presented bestusing the statistics of heel angles, available at MARIN, that are endured by podded propelled vesselswhile manoeuvring, see Figure 8.

Turning circle manoeuvre:Max heel angle

0

10

20

30

0 10 20 30 40Steering angle [deg]

[deg

]

Turning circle manoeuvre:Constant turn heel angle

0

6

12

18

0 10 20 30 40Steering angle [deg]

[deg

]

Zig-zag manoeuvre:Max heel angle (20°/20°)

0

5

10

15

20

25

0 10 20 30 40L/V [s]

[deg

]

Figure 8. Roll angles during zig-zag and turning circle manoeuvres

At high speeds and large steering angles the maximum roll angles can go up to 28 degrees and theconstant turn heel angles up to 17 degrees. The IMO does not provide recommendations regarding rollangles, but maximum roll angles while manoeuvring above 13 degrees and constant roll angles whileturning above 8 degrees are thought to be very large. From the comparison studies, the relationspresented in Figure 9 show how the roll angles during manoeuvres for both concepts correlate.

Maximum heel angles

0

10

20

30

0 10 20 30�max [deg], conventional units

�m

ax [d

eg],

pod

units

Meanline

Constant heel angles

0

6

12

18

0 6 12 18�st [deg], conventional units

�st [d

eg],

pod

units

Meanline

Figure 9. Roll angles while manoeuvring of ships equipped with podded or conventional propulsionunits

Figure 9 shows that in general higher roll angles were measured for the vessels equipped with thepodded propulsion. After thorough analysis of the results the differences in roll angles are judged tobe best attributed to the following non-similarities between the comparable models:� GM value� Difference in hull shape, for example V shaped and extreme pram shape sections in the aft ship� Steering rate of application of rudders or pods

� Force initiated by the steering unit

The heel angle � obtained during turning is related to the instantaneous speed of the vessel U, themetacentric height GM and the turning diameter Dstc. Using basic transverse stability considerationsthis relation can be presented as follows:

stc2

sin g GM D kU

� � � ��

in which g is the gravity acceleration and k an almost constant factor. The applicability of thedescribed relation was evaluated and it has been found out that the k-factors could not be derived aspurely constant, disputing the liability of the relation. The relation however presents the trendsufficiently in order to study the relevant phenomena.In theory, the GM value can be modified for any design without adapting the hull lines by modifyingthe KG value, being the vertical position of the centre of gravity. The influence of the hullshape onthe roll behaviour is related to the following aspects:� The metacentric height can be influenced by the hull shape. Within the same block (LPP·B·T) the

metacentric height can be modified by changing waterline area and the displacement volume.� The drifting and yawing characteristics of the hull form will influence the speedloss and yaw rate

while turning. In the above equation, it can be seen that the instantaneous speed U and the yawrate, inversely related to the turning diameter Dstc, will influence the roll behaviour significantly.

� It is known that a pram shaped vessel has an inherent strong roll-yaw coupling. In comparison to amore V-shaped aft ship a sort of cambered waterline line is observed already at small heel anglesfor extreme pram shaped hull forms. Due to this cambered underwater body a hydrodynamic sideforce and yawing moment are introduced. This coupling will yield a built up of yaw rate and heelangle while turning. The trend is very pronounced for ships with a significant fore and aftasymmetry. Most high-speed ships have a bulbous bow for resistance optimisation and a pramtype aft body. When this is combined with a low GM value, a significant roll and steer couplingwill exist. This issue is applicable to the statistics presented in this paper.

The rate of steering application influences the roll behaviour significantly at each execute. In Figure 8and Figure 9 very large heel angles can be observed occurring at the moment of a rudder or podexecute. The maximum roll angles during a turning circle and a zigzag manoeuvre are very related tothis steering rate. It has also been observed during measurement that a vessel was excited in its naturalperiod of roll due to a higher steering rate, introducing very large roll angles. Especially at highspeeds it is advisable to avoid large heel angles due to a large steering rate. The problem is that thesteering rate should match the classifications required for any azimuthing propulsion gear. A criterionapplicable to tugs as well as high speed vessels should be disputed based on the different sailingcharacteristics and application of both vessel types.

3.4 Course keeping in waves

The course keeping behaviour of vessels equipped with podded propulsion sailing at high speeds incalm water, has been discussed in a previous paragraph. Evaluating the coursekeeping ability underenvironmental loads such as wind but especially waves will add interesting issues to the discussion.

A comparison study has been conducted with a vessel equipped with either a conventional propeller-rudder arrangement or a podded propulsion arrangement. In stern quartering waves it was observedthat the concept with pods had better course keeping capabilities than the conventional propulsionconcept. The steering band that will be used in 95% of the cases was approximately 30% smaller forthe podded propulsion concept. Also smaller course deviations were measured. The absolutedifferences were however judged to be small. It should be noted that the steering rate of the poddedpropulsion concept was higher. The steering unit was therefore more active and able to respondquicker on dynamical wave loads.

4 Manoeuvring in confined waters

4.1 Steering forces at zero speed

Several model tests have been conducted in which the forces generated by pods were compared toforces generated by comparable rudders. In these cases, the ship model has been kept the same and thepods were replaced by rudders or specific aft ship designs were made for the pod configuration aswell as for the propeller-rudder configuration, such that a more realistic comparison was possible.

In the figure below, Figure 10, an example is given of the longitudinal and lateral forces generated onthe ship by the pods or rudders as a function of the steering angle for the bollard pull condition.Because during the bollard pull condition all steering force is generated by the propeller thrust, thelongitudinal force Fx and lateral force Fy have been made non-dimensional by the propeller thrust Tp.

0.0

0.4

0.8

1.2

-80-4004080

PS � [deg] SB

Fx/T

p [-

]

Pod Rudder

-1.0

-0.5

0.0

0.5

1.0

-80-4004080

PS � [deg] SB

Fy/T

p [-

]

Pod Rudder

Figure 10. Longitudinal (left) and lateral (right) forces on the ship

0.0

0.4

0.8

1.2

-80-4004080

PS � [deg] SB

Fx/(T

p·co

s �) [

-]

Pod Rudder

0.0

0.4

0.8

1.2

-80-4004080

PS � [deg] SB

Fy/(T

p·si

n �) [

-]

Pod Rudder

Figure 11. Longitudinal (left) and lateral (right) force coefficients

Based on these graphs the following conclusions can be drawn:� In both cases, the thrust of the propeller is the same. However, it is seen that the longitudinal

forces generated when using a rudder are found to be larger, indicating less thrust deduction forthe ship with rudders. The reason for this is the set-up of the pod and rudder configurations.

� The slope of the curve for the longitudinal force is steeper for the rudder than for the podconfiguration. This indicates a larger drag coefficient for the rudder than for the pod, even thoughthe thrust of the pod propeller is not directed longitudinally anymore. When dividing the non-dimensionalised longitudinal force by the cosine of the steering angle �, Figure 11 presentedbelow is found. In these graphs, it is seen that the forces generated by the pod are almosthorizontal and therefore are directed conform the direction of the propeller thrust.

� The lateral forces generated by the pod are larger than those generated by the rudder.Additionally, stall appears on the rudder at about 35°, while stall on the pod does not occur, due to

the fact that the force is directed in the direction of the propeller thrust. It is seen that at 45° ofsteering angle, the force generated by the pod is about twice the force of the rudder.

� The slope of the curve for the lateral force is steeper for the pod than for the rudder configuration.This indicates a higher "lift coefficient" for the pod than for the rudder.

4.2 Crabbing

For ships such as cruise ships or ferries, the crabbing ability is of major importance for the operabilityand effectiveness of the ship. When the ship is able to berth without any outside assistance, not onlytime is saved but also money in terms of tug fees. Therefore, for these types of ships, the crabbingability should be investigated in the early stages of the design, to verify the bow and stern thrustercapabilities and possibly the design of the superstructure.Quadvlieg and Toxopeus (1998) have given examples of criteria that may be used to judge thecrabbing ability of a ship in the early design stage. Additionally, the standard crabbing experiments asthey are conducted at MARIN are described.

The standard set-ups for crabbing experiments with ships with pods and ships with conventionalpropellers and rudders are visualised in Figure 12. For experiments close to the quay, the side of thebasin is used to model the quay structure. The experiments are in general conducted in three phases:� Captive tests to obtain the forces and moments that can be generated by the devices.� Wind tunnel tests to obtain the forces and moments for each wind direction.� Combining the results of the previous two phases in order to obtain the crabbing ability of the

ship.During the experiments, two modes of operation can be distinguished: berthing or unberthingoperation. In general, it is found that the unberthing mode is the most critical situation.

For conventional ships, onepropeller is set to the so-calledbacking mode, while the otherpropeller is set to the balancingmode, cancelling thelongitudinal force. The rudderbehind the balancing propelleris set to several angles to obtainthe relation between thesteering angle and thegenerated forces and momentson the ship. During severalprojects, it was found that whenoperating close to the quayduring unberthing operations,the best procedure was to setthe quayside propeller to thebacking mode and the otherpropeller to the balancingmode. When using the bowthruster in this case, thepropeller slipstream comingfrom the backing propeller isblocked between the quay walland the side of the ship,generating a pressure field,helping the ship to leave the quay.For ships with pods, more flexibility is available in positioning the angle of both pods. In general, theangle of the quay side pod is varied, while the other pod, running at the same RPM, is used to cancel

Figure 12. Set-up and sign definition for crabbing tests

the longitudinal speed. In general, it is found that the optimum results for unberthing are found whenthe quay-side pod is directed with the trailing edge slightly aft of perpendicular to the quay (between75° and 90° of steering angle) and the other pod directed with the trailing edge slightly forward (atabout 90° to 120°).

When comparing the results of crabbing experiments with conventional steering arrangement andexperiments with pods, in general it is found that the results for ships with pods are much moreconsistent than for conventional ships. This is mainly caused by the strong interaction between theconventional propulsion working in backing-balancing mode, creating a strong current between thequay and the ship. For ships with pods, this interaction is not introduced, which simplifies theoperation of the ship during crabbing manoeuvres considerably.

Additionally, it became clear that for the conventional ship, the best crabbing results are found whenusing almost the complete amount of installed power. When using pods, only a limited amount ofpower is required. For example, some results show that to obtain about the same transverse force incombination with a pure sidewaysmotion (zero yawing moment) about75% of the installed power is requiredfor the conventional ship against about30% for the ship with pods. This notonly means fuel savings, but alsoreduces the impact of the ship on theenvironment, such as quay erosion.

Because of the consistent and straight-forward results of the ship with pods, itis possible to combine the results of thecaptive experiments with wind tunnelresults and obtain the so-called crabbingability footprint, indicating the limitingwind speeds for each direction in whichpure sideways crabbing is possible. InFigure 13, an example is given of such acrabbing ability footprint.

From the example crabbing ability plot,the following observations can be made:� Crabbing in bow or stern winds

poses no problems.� Going to the quay can be done in stronger winds than when leaving the quay.� Going to the quay is possible in winds up to Bf 7, irrespective of the direction of the wind.� Leaving the quay is possible in winds up to Bf 7, except for bow quartering winds.

4.3 Low speed manoeuvring

The use of pods during slow speed manoeuvring differs significantly from the use of conventionalsteering arrangements. The helmsman has the possibility to rotate the pods in all directions and mayuse positive as well as negative propeller revolutions. For twin-podded ships, the steering angles ofthe pods are uncoupled, such that a large number of degrees of freedom is available, possiblyconfusing inexperienced helmsmen. When steering a ship, basically two state variables should becontrolled: the speed of the ship and the heading of the ship.

Therefore, guidelines for operation of podded ships during slow speed manoeuvring were developedin the past using simulator studies. During these studies, the most comprehensive mode of operationof the pods was examined. One possible solution was to control speed and heading independent of

Figure13. Crabbing ability plot for a ship with pods

Bf 9

21.9m/s

Bf 7

15.1m/s

0°10°

20°30°

40°

50°

60°

70°

80°

90°

100°

110°

120°

130°

140°150°

160°170°

180°190°

200°210°

220°

230°

240°

250°

260°

270°

280°

290°

300°

310°

320°330°

340°350°

Leaving the quayGoing to the quay

each other. In general, the ship was sailed with the pods running at the same RPM and positioned atan angle of about 45° with respect to the ship's centreline, with the trailing edges of the pods turnedinward. It was proposed to control the speed of the ship by maintaining pod revolutions, but byreducing the angle of the pods. To control the heading of the ship, the RPM of one pod was increasedwhile the RPM of the other pod was decreased with a corresponding amount. With this approach, theheading of the ship remained constant when controlling the speed and vice versa.During the simulator studies and the subsequent full-scale operation of the ship, it was concluded thatthis approach provided a comprehensive, efficient and safe procedure of sailing the ship at low speedsin confined water.

5 Design guidelines

Based on the assessment of the applicability of podded propulsion in the design of high performancecraft, as described in this paper, design guidelines can be composed. Relevant and significantguidelines are presented in the following paragraphs.

5.1 Hull form design.

As discussed it is observed that currently vessels outfitted with podded propulsion have extreme pramshaped frames in the aft ship. The necessity of these frame shapes is dependent on structuralconsiderations. From a hydrodynamics point-of-view it needs to be further investigated which aft shipshape has the best all-round hydrodynamic characteristics that satisfies the ship design compromise.Manoeuvring, seakeeping and powering assessments should be made in such an evaluation. Forinstance, research indicates that the course keeping ability of V shaped sections in the aft ship tends tobe better than for extreme pram shapes. Compromises to the pod design and innovative ways ofinstallation of the pods could be required. Another aspect of the hulldesign related with the subject is the centre-line skeg design, if applicable.A large skeg is recommended from a course keeping point of view. Turning ability is judged to be noproblem due to the large steering forces. Larger skegs will also reduce large drift angles and yaw ratesdiminishing the heel angles. Disadvantages of the application of large centre-line skegs could be theinterference with the pods at high steering angles. Especially in the crabbing situation the pods can befully shielded by a large skeg. Another steering procedure could be used to avoid this problem. Foreach separate application a compromise should be made to the design of a centre-line skeg betweencourse keeping abilities and crabbing abilities. In general, extending the centre skeg aft to about 2.5%of the length of the ship forward of the aft perpendicular provides sufficient lateral area, withoutcompromising the crabbing ability.

5.2 Implementation of pod units in the hull design.

The way of orientating the pods under the vessel could befurther optimised. In Figure 14 an example is presented oforientating pods under a vessel with V-shape sections. In thisconfiguration the pods will loose some steering efficiency butwill introduce a heeling moment while steering that counteractsthe heeling moment initiated by the gyration and drift forces.The feasibility of such a configuration should be furtherexamined through structural and hydrodynamic research.Significant improvements could be made by exploring thistopic.

5.3 Pod design.

Several pod designs are nowadays spotted on the market according to different concepts unique foreach manufacture. The concepts differ among other aspects, in strut design, number of propellers and

Figure 14:Orientation of pods

presence of nozzles. It is judged that the steering efficiency could be further optimised by adapting thefollowing parts of the unit:� changing the torpedo and strut design� adding fins� adding steerable flaps for course keeping

Complications could be met regarding installation and constructive issues by changing the geometryof the pods. Suitable solutions should be found through constructive as well as hydrodynamicalresearch.

The cavitation behaviour of the podded propulsion system is judged to be critical if pods are appliedat higher speed ranges. Alternative control surfaces could be implemented in the design to avoid largeangles of attack while course keeping and turning at high speed. However, previous research intoadditional rudders or steerable flaps has not proved to be successful up to now. Furthermore,optimisations of the propeller design as well as the strut design could prevent or reduce the occurrenceof cavitation.

When using pods that can not rotate, as is done in some of the current designs of very large cruisevessels, the naval architect should beware of the fact that only part of the available power in the shipwill be used for steering. This may reduce the manoeuvrability considerably compared to ships withall units movable. In these cases further manoeuvring assessments in the early design stages arerequired in order to ensure sufficient manoeuvring capacity of the ship.

5.4 Preventive measures against large roll motions.

In order to prevent large heel angles when steering at high speeds with a vessel equipped with poddedpropulsion unit(s), the following measures should be taken care of:� Provide sufficient intact stability.� Restrict large steering angles and steering rates when sailing at high speeds. Install a steering

control system that only allows large steering angles and steering rates during slow speed oremergency manoeuvres.

� Increase the resistance to drift and yaw by adding course stabilising surface such as a (enlarged)centre-line skeg.

� Explore the possibility of an unconventional orientation of the pods as described in paragraph 5.2.

Furthermore, it is proposed that IMO should provide criteria regarding acceptable heel angles duringmanoeuvring and should require model tests and/or trials to demonstrate compliance with thesecriteria.

5.5 Ensuring sufficient crabbing ability

In order to ensure sufficient crabbing ability, it should be noted that due to the large forces that can begenerated and available power of the pod units, the bow thrusters will be the limiting factor duringcrabbing operations. To reduce the forces in the bow of the ship, the centre of the lateral wind area ofthe ship can be shifted aft.

6 Conclusions

Implementing pods into the ship design potentially increases the performance of the ship in severalareas comprising among others powering and manoeuvring. However, when not taking theappropriate measures, the success of the design with pods is not guaranteed. Already in the earlydesign stage, the naval architect should recognise the areas of concern. From a manoeuvring point ofview, it is found that large heeling angles can occur due to the large steering forces of the pods

compared to conventional rudders. Additionally, due to the up to now rather conventional hull forms,the podded ship might suffer from course instabilities.

Extensive research during the past 10 years has shown that all difficulties can be overcome whenrecognised and dealt with in the early design stages. In this paper, the differences betweenconventional steering arrangements and pods are presented. Design guidelines are given to aid thenaval architect to avoid the problems that are related to the application of podded propulsors.However, although these guidelines will help in avoiding problems during the operation of the ship,hydrodynamic evaluation using detailed calculations or model tests will still be required to avoidunforeseen situations.

References

Hämäläinen, R. and Heerd, J. van (2001). Triple Pod Propulsion in the World’s Largest Ever CruiseLiner. Practical Design of Ships and Other Floating Structures, Shanghai, China.

IMO Resolution A.751 (18) (1993), Interim Standards for Ship Manoeuvrability.

IMO Draft MSC Circular 644 (1994), Explanatory Notes to the Interim Standards for ShipManoeuvrability.

Kijima, K. and Furukawa, Y. (1998). Effect of Roll Motion on Manoeuvrability of Ship. InternationalSymposium and Workshop on Force Acting on a Manoeuvring Vessel, Val de Reuil, France.

Kurimo, R. (1998). Sea Trial Experience of the First Passenger Cruiser with Podded Propulsion.Practical Design of Ships and Mobile Units, The Hague, The Netherlands.

Lepeix, R. (2001). Hydrodynamic Trends in Hull Lines of Podded Driven Large Cruise Vessels.Practical Design of Ships and Other Floating Structures, Shanghai, China.

Oltmann, P. (1993). Roll - An Often Neglected Element of Manoeuvring. International Conference onMarine Simulation and Ship Manoeuvrability, St. John's, Canada.

Pustoshny, A.V. and Kaprantsev, S.V. (2001). Azipod Propeller Blade Cavitation ObservationsDuring Ship Manoeuvring. CAV2001 Fourth International Symposium on Cavitation, Pasadena, USA.

Quadvlieg, F.H.H.A. and Toxopeus, S.L. (1998). Prediction of Crabbing in the Early Design Stage.Practical Design of Ships and Mobile Units, The Hague, The Netherlands.

Son, K. and Nomoto, K. (1981). On the Coupled Motion of Steering and Rolling of a High SpeedContainer Ship. Journal of the Society of Naval Architects of Japan, Vol. 150, 232-244.